The present invention refers to a pipe switch or multiport valve for pneumatically conveying bulk material, and in particular to a pipe switch of the type including a housing provided with a first cylindrical conduit and a second cylindrical conduit with aligned housing ports and with a third cylindrical conduit, with the housing accommodating a cock which is rotatable between rotational positions for selectively connecting the first conduit with the second and third conduits via at least one straight cylindrical flow channel.
A pipe switch of this type is known for example from German patent DE-PS 38 17 349 which includes a cock with two flow channels. What is common to all of these conventional pipe switches is that in the first rotational position of the cock, the axis of the first conduit, the flow channel and the axis of the second conduit are in alignment. Thus, the passageway has a continuous cylindrical cross section. However, in the second rotational position of the cock in which the first conduit is in communication with the third conduit via the same flow channel or a separate parallel second flow channel of the cock, a so-called geometric joint (a juncture of sudden cross sectional change when a first (pipe) cross section is continued by a non-coincidental second (pipe) cross section) is created between the housing port of the first conduit and the opposing port of the respective flow channel of the cock. Such a geometric joint is created because the extension of the cylindrical inside wall surface of the housing with the preceding cylindrical tubular cross section of the conduit and the respective extension of the cylindrical circumferential surface of the cock with the cylinder as described by the flow channel generate sectional surfaces, which, because of the eccentricity of the respective cylinder axes, are asymmetric with regard to the bisecting surface line of the inside surface of the housing and circumferential surface of the cock. The sudden cross-sectional change caused by this asymmetry can be eliminated through refinishing of at least one of the ports. This not only complicates the manufacturing process but also alters the circular configuration of the cross section of the passageway when the cock occupies its first rotational position.
European publication EP 0 406 577 A1 discloses a pipe switch with a cock including one flow channel, in which the creation of a geometric joint in the second rotational position of the cock is avoided by providing the port of the first conduit and the opposing ports of the flow channel of the cock in form of a plane, elliptic sectional surface which, based on the principle of the miter cut, lies upon the bisecting line of an angle defined by the axis of the conduit and the axis of the flow channel. However, in order to attain a smooth juncture between the flow channel of the cock and the second conduit in the first rotational position of the cock, further suitable cross sectional modifications are required in this area. Moreover, the cock can not be made as a one-piece cast part because for manufacturing reasons the provided miter cut surface can only be accomplished by inserting a pipe in the cock.